| Porcine reproductive and respiratory syndrome (PRRS) is considered the most economically important disease of swine through the 1990s. After acute infection, the virus can establish persistent infection in the natural host. During persistence the PRRS virus (PRRSV) seems to be restricted to lymphoid tissues. Although infectious virus has been isolated from persistently infected animals, the extent of viral gene expression in the animal tissue and the genetic stability of the persistent isolates have not been determined. Vaccines have been developed to control the disease. However, little information was available regarding the genomic bases for attenuation.; In this dissertation, the complete genomic sequence of the North American PRRSV 16244B isolate was determined and compared to the European Lelystad virus. Significant differences were found in the non-structural proteins between both PRRSV isolates confirming the presence of distinct North American and European PRRSV genotypes. The attenuation phenotype of a PRRSV vaccine strain was confirmed in the natural host and the genome of the attenuated strain was sequenced and compared to the pathogenic 16244B and VR-2332 PRRSVs to identify putative candidates for attenuation. Four amino acid changes were identified that may provide the basis for the attenuated phenotype. The extent of PRRSV gene expression and frequency of persistence in infected animals was analyzed by virological and molecular assays based on minus-strand specific RT-PCR. The negative RNA form was detected in tonsil tissue up to 119 days post infection (dpi) and the infectious virus was still present in a small fraction (2/10) of animals at 150 dpi. Furthermore, the genetic stability of the persistent isolates varied at a different rate depending on the region of PRRSV genome analyzed. It was highly stable in the 3' and 5' non-coding regions and in the viral nucleoprotein while mutations were observed in the glycoproteins and M protein. These mutations could be associated with selection of a viral subpopulation and putative changes in tissue tropism addressed to maintain persistence. |
